IDENTIFYING ENVIRONMENTAL PREDICTORS AND QUANTIFYING METHANE AND CARBON DIOXIDE FLUX FROM FOUR VERNAL POOLS, MAINE USA

Vernal pools are small seasonal wetlands that are a common landscape feature of significant ecological importance in northeastern North American forests. However, even basic information about their biogeochemical functions, such as carbon cycling, is largely unknown. In this study, we estimated greenhouse gas fluxes from four vernal pools in central and eastern Maine, USA, with distinctly different geologic substrates and hydroperiods. Dissolved gas concentrations (CH₄, CO₂) and other water chemistry parameters (i.e. ortho-P, NH₄⁺, SO₄^2-) were monitored weekly at the bottom and surface of each pool between April and August 2016. The vernal pools were supersaturated with respect to CH₄ and CO₂ at all sampling dates and locations. Concentrations of dissolved CH₄ and CO₂ ranged from 0.4 to 2.1´10² μmol L^-1 and 72 to 2.3´10³ μmol L^-1, respectively. Evaporative fluxes of CH₄ and CO₂ into the atmosphere ranged from 0.2 to 73 mmol m^-2 d^-1, and 30 to 5.9´10² mmol m^-2 d^-1, respectively; CH₄ and CO₂ flux increased over the season. Mass balance calculations showed that the pools had a relatively small capacity for gas storage, and that the produced gases evaporated into the atmosphere. During the study period, the vernal pools emitted between 0.2 to 5.9 kgC m^-2 and 11 to 1.4´10² kgC m^-2 of CH₄ and CO₂, respectively. Both CH₄ and CO₂ were positively correlated with surface temp, ortho-P, NH₄⁺, DOC, alkalinity, and precipitation, and negatively correlated with pool volume, surface area and depth. Methane concentrations were best predicted by alkalinity and temperature. Carbon dioxide concentrations were best predicted by alkalinity, temperature, and dissolved oxygen. Carbon dynamics and greenhouse gas emissions are key to understanding how vernal pools fit into the global carbon budget and how their carbon cycles compare to other wetlands.